Magnetic storage systems, such as a hard disk drive (HDD), are utilized in a wide variety of devices in both stationary and mobile computing environments. Examples of devices that incorporate magnetic storage systems include desktop computers, portable notebook computers, portable hard disk drives, digital versatile disc (DVD) players, high definition television (HDTV) receivers, vehicle control systems, cellular or mobile telephones, television set top boxes, digital cameras, digital video cameras, video game consoles, and portable media players.
A typical HDD includes magnetic storage media of one or more flat disks. The disks are generally formed of two main substances, namely, a substrate material that gives it structure and rigidity, and a magnetic media coating that holds the magnetic impulses or moments that represent data. A HDD also typically includes a read head and a write head, generally a magnetic transducer which can sense and/or change the magnetic fields stored on the disks. Perpendicular magnetic recording (PMR) involves recorded bits that are stored in a generally planar recording layer in a generally perpendicular or out-of-plane orientation. A PMR read head and a PMR write head are usually formed as an integrated read/write head on an air-bearing slider. In a PMR reader, a tunnel magnetoresistance (TMR) sensor is frequently employed in the read head.
A TMR sensor includes a patterned TMR structure or stack having two ferromagnetic layers separated by an insulating barrier layer (e.g., MgO). One ferromagnetic layer is magnetically oriented in a fixed direction (the “pinned layer”) and the other ferromagnetic layer rotates in response to an external magnetic field (the “free layer”). The TMR sensor also typically includes a hard bias layer disposed on either side of the TMR stack. The hard bias layer includes a permanent magnetic material, such as cobalt platinum (CoPt), and provides a bias field along a direction perpendicular to layers of the TMR stack. The resistance of the device is dependent on the relative orientation between the two ferromagnetic layers. In a TMR read head, a sense current passes perpendicularly through layers of the TMR stack. The magnetic transitions between adjacent oppositely-directed magnetized regions cause changes in electrical resistance that are detected by the TMR sensor.
The amplitude of a readback signal of a PMR reader can be asymmetric. Readback signal amplitude asymmetry indicates that the amplitude of the pulses from magnetizations recorded in one direction (e.g., the “positive” direction) is different from the amplitude of the pulses from magnetizations recorded in the opposite direction (e.g., the “negative” direction). The amplitude asymmetry (AASY) measured in percent may be expressed as [(SP−SN)/(SP+SN)]*100, where SP represents the measured amplitude of the pulses from magnetizations recorded in one direction and SN represents the measured amplitude of the pulses from magnetizations recorded in the other direction. A high value of AASY is undesirable since it has a deleterious effect on the stability of the read head by causing a high bit error rate (BER) when the data is read back. AASY is partly a result of the construction of the reader. Spurious magnetic fields arising from the media background and other sources also contribute to amplitude asymmetry. There is a current demand for increasing track density, requiring decreasing reader track width. However, as reader track width is decreased, de-magnetic fields rapidly increase, and the increased de-magnetic field in turn causes reader stability degradation by increasing the AASY.
Conventionally, in an effort to improve reader stability, the thickness of the hard bias layer is increased. However, with increasing track density, there is a requirement to reduce a shield-to-shield spacing for the TMR sensor stack. Therefore, it is often neither desirable nor practical to increase the hard bias layer thickness to improve the AASY. In addition, an increase in the hard bias thickness causes a significant reduction in the reader amplitude.